Читать книгу Cyber-physical Systems - Pedro H. J. Nardelli - Страница 30

This section deals with an interesting problem of thermodynamics, a field of physics defined as follows [8]: science of the relationship between heat, work, temperature, and energy. In broad terms, thermodynamics deals with the transfer of energy from one place to another and from one form to another. The key concept is that heat is a form of energy corresponding to a definite amount of mechanical work. Among its fundamental laws, the first and second ones will be introduced here in brief. The first is the law of conservation of energy, which states that the change in the internal energy of a system is equal to the difference between the heat added to it from its respective environment and the work done by the system on its respective environment. The second law of thermodynamics asserts that entropy (which, in very rough terms, quantifies the degree of organization) of isolated (closed) systems (i) can never decrease over time, (ii) is constant if, and only if, all processes are reversible, and (iii) spontaneously tends to its maximum value, which is the thermodynamic equilibrium.

Such fundamental laws of physics were postulated in the nineteenth century when the study of heat transfer and heat engines was widespread because of the needs of the industrial revolution. As discussed in the previous chapter, this is another example of how relatively autonomous scientific knowledge can emerge from technical needs determined by a specific socioeconomic conjuncture [9]. This relative autonomy of the theory allows scientists to pose interesting thought experiments. One of the most famous is Maxwell's demon, in which the second law of thermodynamics would hypothetically be violated [10]. This experiment is defined next.

Definition 2.4 Maxwell's demon [11] Maxwell's demon, hypothetical intelligent being (or a functionally equivalent device) capable of detecting and reacting to the motions of individual molecules. It was imagined by James Clerk Maxwell in 1871, to illustrate the possibility of violating the second law of thermodynamics. Essentially, this law states that heat does not naturally flow from a cool body to a warmer; work must be expended to make it do so. Maxwell envisioned two vessels containing gas at equal temperatures and joined by a small hole. The hole could be opened or closed at will by “a being” to allow individual molecules of gas to pass through. By passing only fast‐moving molecules from vessel A to vessel B and only slow‐moving ones from B to A, the demon would bring about an effective flow from A to B of molecular kinetic energy. This excess energy in B would be usable to perform work (e.g. by generating steam), and the system could be a working perpetual motion machine. By allowing all molecules to pass only from A to B, an even more readily useful difference in pressure would be created between the two vessels.

Figure 2.1 depicts the situation. Our goal in this subsection is to analyze this problem as a system, indicating its (theoretical) conditions of existence and its PF. The idea here is not to solve this conundrum but rather analyze it as a system to properly define the problem and its characteristics. This should clear our path to theoretically work on the problem and then produce more knowledge about this object. Note that Maxwell's demon will also be studied in future chapters, and thus, it is worth for the reader to familiarize with it.

Figure 2.1 Illustration of the Maxwell's demon thought experiment.